This invention generally relates to radar absorbing material (RAM). More specifically, the present invention relates to particulate RAM formed from a ferromagnetic core surrounded by a protective shell that shields the core from oxidation. The invention further relates to methods by which to produce such particulate RAM.
Powders formed from ferromagnetic particles are widely used in radar absorbing applications. In particular, ferromagnetic materials characterized by poor conductivity (also referred to as lossy-dielectrics) are known for their ability to absorb microwave energy. Typically, such ferromagnetic particulates are incorporated into attenuating coating compositions that are spray painted onto the surface of a substrate to reduce its electromagnetic signature. It is known in the art that more efficient radar absorbing coating compositions approach the density provided by solid ferromagnetic materials, such as iron. Therefore, it is considered beneficial to employ the minimum amount of resin possible to provide cohesion to the coating and yet maintain separation among the ferromagnetic particles. It is important that the particles within the radar absorbing coating composition be kept separate because agglomerates of ferromagnetic particles become increasingly conductive, thus diminishing the ability of the coating composition to absorb microwave energy. Therefore, ferromagnetic particles having a non-conductive coating and which are further characterized by a uniform particle size distribution would be highly beneficial for use in radar absorbing coating compositions.
Iron has been used extensively as a ferromagnetic material in radar absorbing materials (RAMs), primarily because it provides both adequate shielding properties and is cost effective. However, the use of iron in either its pure or carbonyl form is generally considered problematic due to iron""s tendency to form rust under normal environmental conditions. Rust causes the iron particles to lose their magnetic properties, and is therefore detrimental to shielding properties. Protective binders have been used to prevent rust, but most are not effective in protecting the RAM against the aggressive environments commonly encountered during use, such as salt spray environments and temperature extremes.
In alternative efforts to increase corrosion resistance, iron silicide alloys have been produced in powdered forms and used as RAM. Iron silicide powders do have improved corrosion resistance; however, their radar absorbing performance per weight and amount of iron is substantially less than that of either pure iron or carbonyl iron powders. Iron silicide coatings are noted in U.S. Pat. No. 4,137,361, in which conductive coatings are applied to iron particulates. Further, hybrid iron silicide particles, formed by diffusing silicon into carbonyl iron particulates, are also known, such as those disclosed in U.S. Pat. No. 5,866,273, hereby incorporated by reference in its entirety. However, the magnetic properties of carbonyl iron are superior to those of the bulk iron silicide particles provided by such hybrid structures. Further, the method used to produce such hybrid iron silicide particles forms a sintered mass that requires grinding prior to incorporation into coatings and the like. In addition to the added expense incurred by grinding, iron silicide is a more brittle material than pure or carbonyl iron, and thus the a mass of ground particles would be expected to provide a less uniform particle distribution.
Therefore, there remains a need in the art for ferromagnetic particles, in particular iron particles, having non-conductive coatings, particularly nonconductive coatings that further provide corrosion resistance. There is also a continuing need in the art for coated ferromagnetic particles having a uniform particle distribution which are additionally cost effective.
The present invention provides ferromagnetic particles having a non-conductive coating, for use in electromagnetic shielding applications and the like. In particular, the present invention provides ferromagnetic particles having a protective coating that provides both insulating properties and corrosion resistance to the particle without sacrificing the electromagnetic characteristics of the ferromagnetic material. As a result of the insulating characteristics imparted by the non-conductive coatings of the present invention, the robustness of RAM coating compositions employing conventional quantities of ferromagnetic filler is improved. Further, the non-conductive coatings of the present invention allow the use of higher loadings of ferromagnetic particles in RAM coating compositions, without inducing the conductivity issues normally encountered at comparable loadings of conventional ferromagnetic particles. The ferromagnetic particles of the present invention are further characterized by a uniform particle distribution and are produced without the need for additional grinding processes. This is particularly advantageous because it preserves the shape of the core particle. Thus, coated particles of the present invention may be provided in a variety of useful shapes, such as spheres, flakes, and fibers.
In one particularly advantageous embodiment, a coated particle capable of withstanding corrosive environments is provided which comprises a core formed from one or more layers of ferromagnetic material surrounded by a protective shell formed from one or more layers of a non-conducting material. In this advantageous embodiment, the protective shell is deposited onto the outer surface area of the core such that the protective shell shields the core from oxidation and further forms a substantially continuous non-conducting layer. The core may be comprised of a variety of ferromagnetic materials, including iron, carbonyl iron, cobalt, nickel, or alloys thereof. The protective shell may likewise be comprised of a variety of non-conducting materials, e.g. materials having a resistivity of grater than 2500 ohm-cm, including silicon, silicon oxide, chromium oxide, aluminum oxide, and mixtures thereof. Further, in advantageous embodiments of the present invention, the particulate core is substantially free of the non-conducting material that forms the protective shell, thereby providing a coated ferromagnetic particle whose shielding properties are comparable to an uncoated particle formed from the same ferromagnetic material. In one particularly advantageous embodiment, the protective shell has a thickness ranging from about 0.05 to about 20 microns. Coating compositions incorporating the coated particle of the present invention are also provided.
In additional advantageous aspects, the protective coating may be applied to the ferromagnetic particles of the present invention using a retort mounted about a generally horizontal axis of rotation. In such an advantageous embodiment, the retort is partially filled by placing a sufficient quantity of ferromagnetic particles into the empty retort, which is subsequently placed under vacuum. Following evacuation of the retort, a gaseous composition comprising one or more active elements to be deposited onto the surface of the ferromagnetic particles is introduced into the retort. In one preferred embodiment, the active elements contained within the gaseous composition are selected from the group consisting of silicon, chromium, aluminum, and oxygen. The use of a combination of silicon and oxygen as active elements is particularly advantageous because the silicon dioxide coating produced by such a combination has a low dielectric constant. Gases containing a combination of silicon and oxygen may be derived from, for example, silanes such as triethoxysilane, trimethoxysilane, and the like. Alternatively, in an additional beneficial embodiment, a further chemical treatment, such as an oxidation step or other chemical sealing step, may be applied to the coated particles to provide the desired continuous non-conductive coating. During and subsequent to introduction of the gaseous composition, the retort is rotated at a speed sufficient to fluidize the ferromagnetic particles and the temperature within the retort is elevated sufficiently to effect the deposition of the active element onto the ferromagnetic particles.
The present invention thus provides ferromagnetic particles coated with a protective shell. The protective shell is non-conductive, and thus provides insulative properties to the particle. In addition, the protective shell may impart corrosion resistance to the coated particle, without sacrifice to its radar shielding properties. The present invention further provides methods by which such coated ferromagnetic particles are formed.
Further understanding of the processes and systems of the present invention will be understood with reference to the brief description of the drawings and detailed description which follows herein.